The present invention generally relates to the field of environmental control for performing cryogenic spray cleaning processes. More specifically, the present invention is directed at cleaning or treating miniature electromechanical device surfaces with cryogenic impingement sprays.
Conventional precision cleaning processes using cryogenic particle impingement sprays, such as solid phase carbon dioxide, require control of the atmosphere containing a treated substrate to prevent the deposition of moisture, particles or other such contaminants onto surfaces during and following cleaning treatments. Environmental control is required because of localized atmospheric perturbations created by the low temperatures and high velocities which are characteristic of these impingement cleaning sprays.
For example, snow particles having a surface temperature of −100 F and traveling through the space between a spray nozzle and a substrate are continuously sublimating in transit and upon impact with the substrate. This rapidly lowers local ambient atmospheric temperature causing contaminants contained therein to condense or “rain-out” of the local atmosphere and onto treated substrate surfaces during or following spray treatments. Moreover, by way of the Bernoulli effect, the cleaning spray stream exhibits lower internal pressure than the surrounding atmosphere which creates venturi currents adjacent to the flow of the stream. These venturi currents cause the local atmosphere surrounding the stream to collapse into the spray stream above the substrate, thus entraining and delivering a mixture of cleaning spray and atmospheric constituents to the substrate. Finally, static charge build-up and accumulation are common to cryogenic sprays due to dielectric and triboelectric characteristics. This presents problems including, for example, potential device damage from electrostatic overstress or electrostatic discharge, and attraction of atmospheric contaminants to treated substrates via electrostatic attractive forces.
Micro-environmental control technology is well established and many techniques have been developed over the years to isolate either a process, a substrate or a worker. The purpose of isolation generally includes protecting workers from toxic chemicals, protecting clean rooms from particles, or protecting delicate processes and substrates from the outside environment.
There are many examples of techniques to control thermal and electrostatic effects during cryogenic impingement sprays using secondary heated or ionized jets or sprays above the substrate surface and delivered either independently or as a component of the cryogenic spray have been used commercially. For example, U.S. Pat. No. 5,409,418 issued to Krone-Schmidt et al. and U.S. Pat. No. 5,354,384 issued to Sneed et al. suggest direct heated or ionized gas impingement techniques and apparatus for heating, purging and deionizing substrate surfaces. The '384 patent suggests the use of a heated gas, such as filtered nitrogen, to provide a pre-heat cycle to a portion of a substrate prior to snow spray cleaning and a post-heat cycle to the substrate following the snow cleaning. This approach relies on “banking heat” into the substrate portion prior to cryogenic spray cleaning by delivering a heated gas stream to a portion of substrate to prevent moisture deposition and adding heat from a heated gas following cryogenic spray treatment. The '384 patent is primarily useful for removing high molecular weight materials such as waxes and adhesive residues having weakened cohesive energy from surfaces by partially melting or softening them prior to spray treatment. However, the approach of the '384 patent does not work well for most substrate treatment applications because many materials being cleaned, or at least portions thereof, have low thermal conductivity, low mass or because highly thermal conductive materials rapidly lose heat to the sublimating snow during impact. This tends to create localized cold spots on even a mostly hot bulk substrate. Examples of such substrates include ceramics, glasses, silicon and other semi-conductor materials, as well as most polymers. Additionally, many electromechanical devices being cleaned are relatively small, providing no appreciable mass for storing heat. Such examples include photodiodes, fiber optic connectors, optical fibers, end-faces, sensors, dies, and CCD's, among many others.
Most significantly, directing a heating spray, or any secondary fluid for that matter, directly at or incident to the substrate surface during and/or following cryogenic cleaning spray treatments causes the entrainment, delivery and deposition of atmospheric contaminants as discussed above. This necessitates housing the cryogenic spray applicator, substrate and secondary gas jets in large, bulky and complex environmental enclosures employing HEPA filtration and dry inert atmospheres, such as included in U.S. Pat. No. 5,315,793, issued to Peterson et al.
In the '418 patent, an apparatus is taught for surrounding the impinging cryogenic spray stream with an ionized inert gas. It is proposed that by surrounding a stream of solid-gas carbon dioxide with a circular stream of ionized gas and applying the two components to the substrate simultaneously controls or eliminates electrostatic discharge at the surface during impingement. However, as also suggested by the '384 patent, the '418 patent suggests secondary stream that entrains, delivers and deposits atmospheric contaminants upon the substrate surfaces being treated. Moreover, contact of the ionizing gas with the stream prior to contact with the surface rapidly eliminates ion concentration and is ineffective in controlling electrostatic dishcarge. Still moreover, using the ionizing spray of the '418 patent independent of the snow spray and which is directed at an angle incident to the surface will further re-contaminate the substrate unless, as taught in the '793 patent, the entire operation is performed in a controlled HEPA filtered chamber.
As devices become smaller and their complexity increases, it is clearly desirable to have a improved processing technique, including a method and apparatus, that enables the use of environmentally safe cleaning agents to remove unwanted organic films and particles. It is desirable to have a technique which prevents additional particles and residues from being deposited on critical surfaces during application of said impingement cleaning sprays. The complete environmental control technique should include all of the basic environmental controls of thermal control, ionization control, and providing a dry and particle free cleaning atmosphere, but not negatively impacting the performance of the impinging cleaning spray. Moreover it would be highly desirable to have a cleaning capability integrated with the aforementioned controlled environment which provides a compact in-line or bench-top critical cleaning solution for manufacturing operations.